Old Ham Radio Stuff

Recently, someone asked me about a VHF SWR meter, which I built over a decade
ago from widely available parts. I was lucky to find it in a dark corner
of my harddisk, and decided that some of the old drawings may be useful today...
so I put them here, on this website, in the hope it may be useful for someone.

Some of the text in the images is in German language. Lengths are given im
millimeters if not otherwise stated.

VHF SWR Meter from widely avaiable parts

Some notes about this project:

I used a square outer conductor, folded from copper or brass, held in place
by the BNC connector (the square-flange type, called "Flanschbuchse" in my
German). With a 9-mm brass tube as the center conductor, this gives a good
50 Ohm match. The two reflectormeter lines ("Reflektometer" in german) are
1.5 mm OD enamelled copper. The terminator resistors and schottky diodes
are threaded through the former screw holes of the connectors, which just
gives the right distance from the surface of the outer connector. The BAT41
diodes can be replaced by something "better" (with less capacitance).

If you want to use a round outer connector, and found some old brass tubes
in your junk box, here is a formula to calculates the impedance of a coaxial
line, where 'd' is the *outer* diameter of the *inner* tube, and 'D' is the
*inner* diameter of the *outer* tube:

For a SQUARE outer conductor (like in the SWR meter shown above), i had found
a diagram which suggested a D/d ratio of 2.0 for a 50 Ohm impedance. Here:
D = 19 mm (inner diameter of the outer square), and d = 9 mm as outer diameter
of the inner tube.

To improve the accuracy at lower power levels, I added a passive circuit
to bias the diodes a bit (100..200 mV). This helps especially at power levels
from a hand held transceiver (for example when you want to tune your HB9CV
antenna at 1 Watt TX power, and the VSWR is already "quite good".

HF SWR Meter

I integrated this circuit in my "Annecke" antenna tuner, using a small stereo
level indicator ("Doppelinstrument"). The number of turns on each of the
two cores (Ringkerne) must be equal. Both cores are actually used as HF
transformers, with a 30:1 or 20:1 ratio. With a high-permeability core, the
same circuit also works on 136 kHz. Note the different direction of the wires
going through the voltage transformer ("Sekundärspule").

Active analog audio filter (for morse code = "CW")

I still use this old filter today, after re-tuning it from 200 Hz bandwidth
to about 100 Hz. This can be achived simply by moving the center frequencies
of the 4 audio peak filters closer to each other. You don't need a DSP for
this..